docs: clarify hookimpl specname limitation

[MukundaKatta]

Add a note to the hook ordering documentation explaining that @pytest.hookimpl(specname=...) still requires implementation function names to start with pytest_. This documents the current limitation discussed in gh-11307 and points users toward suffixed hook names such as pytest_collection_modifyitems_tryfirst.

Closes #11307

Verification:

• sphinx-build -W --keep-going -b html doc/en doc/en/_build/html
• git diff --check